An organization that provides access to a communication media, such as a telephone system or the Internet, is said to be a Common Carrier if it exercises no controls over the content of that media.
This has important legal implications, since offensive or otherwise illegal material may be carried on any media. If an organization acts as a Common Carrier, then that organization can reasonably expect to have no liability for the contents of that media. Otherwise, an organization that exercises even a very few controls over the content of a medium, such as an Internet connection, does not qualify as a Common Carrier, and may be liable for everything on that medium.
To avoid such liability problems, many organizations, especially including Internet Service Provders, elect to behave as Common Carrier's. This may well be advisable for corporations and government organizations.